Due to their desirable physical properties such as strength and toughness (i.e., the combination of elongation and impact strength), rubber-reinforced polymer resins are employed in a variety of commercial applications. A deficiency of rubber-reinforced plastic materials made from these resins is their vulnerability to environmental conditions, which causes yellowing of the plastic. In certain applications exposure to a light source having as a component ultraviolet (UV) light, e.g., sunlight or fluorescent lighting, can detract from the appearance of plastic parts by causing discoloration of the resin surface, which is commonly referred to as weathering.
Yellowing of rubber-toughened plastic is particularly critical in markets such as business machine housings and parts. The conversion from metals to plastics has resulted in rubber-reinforced plastic components such as business machine housings, which are vulnerable to the degradative effects of fluorescent lighting and sunlight.
The UV stability of thermoplastic business machine housings is receiving close scrutiny from suppliers of multiple component personal computer systems. By using a resin having good UV stability, a manufacturer is assured that the housing of any peripheral component, e.g., a printer or additional disc drive, added at a later date will match the color of the housing of the original equipment. Consistent color among different business machine housings gives the consumer a strong perception of quality.
Interest in UV stability is also being fueled by pressures to make every aspect of business machines as cost effective as possible. In many cases, this is necessitating a switch from the use of structural foam with a highly UV-stable coating to the use of injection molded housings with integral color for equipment housings.
Before UV can cause any harm, it must first be absorbed. Only certain groups within polymer molecules, called chromophores, accept the energy of the UV light and are transformed into excited state groups. These groups then dispose of the energy. The energy may be transferred to a nearby stabilizer molecule called a quencher, which in turn converts the energy to heat or, less desirably, may break weak chemical bonds with minimal color change.
Limited stabilization can be achieved by several conventional mechanisms. UV absorbers operate largely by competitive absorption. Absorbers convert the absorbed energy into harmless heat. Thus, much less light reaches chromophores in the substrate.
An ideal UV absorber should be extremely photo-stable and have high absorption over the entire UV range from 290 to 400 nanometers (nm).
The 2-hydroxyphenyl benzotriazoles are one class of UV absorbers. The benzophenones are another important and widely used class of UV absorbers whose absorption covers mainly the lower half of the UV range. Products of this latter class tend to be more prone to yellowing under processing or light exposure conditions than 2-hydroxyphenyl benzotriazoles. A third class of UV absorbers include rutile TiO.sub.2 metal oxides such as pigment grade titanium dioxide. The benefits of TiO.sub.2 are believed to be more than just as a UV absorber. Higher TiO.sub.2 concentration in a resin increases the sample opacity thereby hindering the observation of discoloration deeper within the material.
Other clases of UV absorbers, which absorb primarily at the low wavelength end of the UV range, include salicylates, cyanoacrylates, benzylidene, malonates, and oxalarrilides. These are generally less effective than UV absorbers in the first three classes.
Hindered amine light stabilizers (HALS) provide another approach to UV stabilization. These molecules, typically derivatives of tetramethylpiperidines, do not absorb UV light, but are effective scavengers of free radicals, thus acting as photooxidation inhibitors. Synergistic enhancement of stabilizing activity is often achieved by simultaneous use of scavengers and stabilizers, which each act by different mechanisms. When choosing a plastic composition, end users in the past have had to make significant expenditures to achieve ultraviolet light stabilization, or accept an inevitable amount of discoloration. Improved stabilization has been achieved using a combination of TiO.sub.2 and a HALS additive. However, due to the cost of the HALS additive, this stabilization combination entails significant expense.
As previously mentioned, quenchers interact with excited states of chromophores to accept the energy transferred and to return the excited chromophore to the stable ground state. Typical quenchers are nickel chelates which can accept energy from excited chromophores. In addition, some of the protective action of this class has been attributed to their hydroperoxide decomposing and radical scavenging ability.
In view of these problems in achieving color stabilization of rubber-reinforced resins, it remains highly desirable to provide an inexpensive additive for rubber resins which minimizes an overall color shift upon exposure of the resin composition to UV light.